In air conditioning, heating and ventilating work, it is helpful to understand the techniques used to determine air velocity. In this field, air velocity (distance traveled per unit of time) is usually expressed in feet per minute (FPM). By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. Volume flow is usually measured in cubic feet per minute (CFM). These volume measurements can often be used with engineering handbook or design information to reveal proper or improper performance of an airflow system.
To move air, fans or blowers are usually used. They work by imparting motion and pressure to the air with either a screw propeller or paddle wheel action. When force or pressure from the fan blades causes the air to move, the moving air acquires a force or pressure component in its direction of motion due to its weight and inertia. This force is called velocity pressure. It is measured in inches of water column (w.c.) or water gage (w.g.). In operating duct systems, a second pressure is always present. It is independent of air velocity or movement. Known as static pressure, it acts equally in all directions. In air conditioning work, this pressure is also measured in inches w.c.
In pressure or supply systems, static pressure will be positive on the discharge side of the fan. In exhaust systems, a negative static pressure will exist on the inlet side of the fan. When a fan is installed midway between the inlet and discharge of a duct system, it is normal to have a negative static pressure at the fan inlet and positive static pressure at its discharge.
Total pressure is the combination of static and velocity pressures, and is expressed in the same units. It is an important and useful concept to use because it is easy to determine and, although velocity pressure is not easy to measure directly, it can be determined easily by subtracting static pressure from total pressure. This subtraction need not be done mathematically. It can be done automatically through the configuration of the instrument.
For most industrial and scientific applications, the only air measurements needed are those of static pressure, total pressure, and temperature. With these measurements, air velocity and volumetric flow can be calculated. To facilitate these measurements in commercial or industrial ductwork, the duct walls can be outfitted with one or more static pressure taps. The pressure taps extend through the duct wall perpendicularly to the air flow direction in the duct and allow for the attachment a pressure sensing device.
For ductwork with substantially uniform, well developed flow, the tap or opening of the pressure sensors can be mounted flush with the wall of the duct. For ductwork where flow is non-uniform and/or turbulent, sensing pressure is not so simple. This may be the case, for example, when sensing static pressure across industrial air filters and refrigerant coils in commercial or industrial ductwork because those structures create disturbances in the air flowing over them. disruptions can also occur in the area of T-junctions where HVAC runs branch off from other ducts. Disruptions in the air stream increases the risk of impingement, aspiration, or unequal distribution of the moving air in the area close to the duct wall. To account for this, when taking static pressure measurements across industrial air filters and refrigerant coils, the use of a static pressure tip is ideal.
Static pressure tips are tubular probe instruments that can be inserted into the duct through a static pressure tap. The static pressure tip extends into the duct perpendicular to the air flow direction and can be positioned away, e.g., centrally, from the duct walls. The static pressure tip has a 90-degree bend near the end, which positions the tip facing the directly into the airflow. Static pressure is sensed through holes drilled radially though the tubular sidewall near the tip of the probe. Thus, instead of measuring static pressure through an opening that is flush with the duct wall, perpendicular to the airflow direction, static pressure is measured through an opening that is flush with the sidewall of the probe, perpendicular to the airflow direction.
In sensing static pressure, it is desirable to eliminate the effect of air movement. Eliminating the effect of air movement requires the determination of velocity pressure fully and accurately. This is usually done with an impact tube which faces directly into the air stream. This type of sensor is frequently called a “total pressure pick-up” since it receives the effects of both static pressure and velocity pressure. Thus, measuring velocity pressure typically involves measuring total pressure pick-up and compensating or cancelling out the effects of static pressure. This can be done by connecting a static pressure sensing device and a total pressure sensing device across a differential pressure manometer. Since the static pressure is applied to both sides of the manometer, its effect is cancelled out and the manometer indicates only the velocity pressure. To translate velocity pressure into actual velocity requires either mathematical calculation, reference to charts or curves, or prior calibration of the manometer to directly show velocity.
In practice, these types of measurements are usually made with a Pitot tube which incorporates both static and total pressure measurements in a single unit. Essentially, a Pitot tube consists of an impact tube (which receives total pressure input) fastened concentrically inside a second tube of slightly larger diameter which receives static pressure input from radial sensing holes around the tip. The air space between the inner and outer tubes permits transfer of pressure from the sensing holes to the static pressure connection at the opposite end of the Pitot tube and then, through connecting tubing, to the low or negative pressure side of a manometer. When the total pressure tube is connected to the high pressure side of the manometer, velocity pressure is indicated directly.
To ensure accurate velocity pressure readings, the Pitot tube tip must be pointed directly into (parallel with) the air stream. As the Pitot tube tip is parallel with the static pressure outlet tube, the latter can be used as a pointer to align the tip properly. When the Pitot tube is correctly aligned, the pressure indication will be maximum. To assure well developed flow, the Pitot tube should be inserted at least 8.5 duct diameters downstream from elbows, bends or other obstructions which cause circulation. In other words, the reading should be taken a distance downstream that is equal to or greater than the 8.5 times the duct diameter. Additionally, to ensure the most precise measurements, straightening vanes should be located 5 duct diameters upstream from the Pitot tube.
In most if not all practical situations, the velocity of the air stream is not uniform across the cross section of a duct. Friction slows the air moving close to the walls, so the velocity is greater in the center of the duct. To obtain the average total velocity in ducts of 4″ diameter or larger, a series of velocity pressure readings must be taken at points of equal area. A formal pattern of sensing points across the duct cross section is recommended. These are known as traverse readings.
For round ducts, it is recommended that velocity pressure readings should be taken at centers of equal concentric areas along two diameters. At least 20 readings should be taken. In rectangular ducts, a minimum of 16 and a maximum of 64 readings are taken at centers of equal rectangular areas. Actual velocities for each area are calculated from individual velocity pressure readings. This allows the readings and velocities to be inspected for errors or inconsistencies. The velocities are then averaged and used to determine flow according to known and accepted practices.
For maximum accuracy, it can be recommended that the following precautions should be observed when taking traverse pressure readings:                The duct diameter should be at least 30 times the diameter of the Pitot tube.        The Pitot tube should be located in a duct section that provides 8.5 or more duct diameters upstream of the Pitot tube and 1½ or more diameters downstream of Pitot tube that is free of elbows, size changes, or obstructions.        An egg-crate type flow straightener should be positioned 5 duct diameters upstream of Pitot tube.        The technician should perform a complete, accurate traverse.        In small ducts, or where traverse operations are otherwise impossible, an accuracy of ±5% can be achieved by placing Pitot tube in center of duct. The air velocity determined from this reading can then be multiplied by 0.9 for an approximate average.        
Manometers for use with a Pitot tube are offered in a choice of two scale types. Some are made specifically for air velocity measurement and are calibrated directly in feet per minute. These calibrations are correct for standard air conditions, i.e., an air density of 0.075 lb per ft3 corresponds to dry air at 70° F., at a barometric pressure of 29.92 inches Hg. To correct the velocity reading for other than standard air conditions, the actual air density must be known. It may be calculated if relative humidity, temperature, and barometric pressure are known. Most manometer scales are calibrated in inches of water. Using readings from such an instrument, the air velocity may be calculated using the basic formula:
      v    =                                        h            v                    d                    ⁢              {                              =                          4004.4              ⁢                                                h                  v                                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              .075              ⁢                                                          ⁢              lb              ⁢                              /                            ⁢                              ft                3                            ⁢                                                          ⁢              dry              ⁢                                                          ⁢                              air                ⁢                                                                  @                                                                  ⁢                70                            ⁢              °              ⁢                                                          ⁢                              F                .                                              ,                                          ⁢                      29.92            ⁢                                                  ⁢                          in              .                                                          ⁢              Hg                                      }              ;where:
v=velocity in feet per minute;
hv=velocity pressure in inches of water; and
d=density of air in pounds per cubic foot.
To determine dry air density, use the formula:
      d    =          1.325      ·                        P          B                T              ;where:
d=Air density in pounds per cubic foot;
PB=Barometric (or absolute) static pressure in inches of mercury; and
T=Absolute temperature (indicated temperature in ° F. plus 460°).
With dry air at 29.9 inches mercury, air velocity can be read directly from standard curves found in HVAC engineering handbooks. For partially or fully saturated air, further correction is required. This can be done using standard curves for air at certain saturation levels, either directly or through interpolation. To save time a variety of tools, such as physical, slide rule type devices or computerized calculators (e.g., smartphone app) can be used.
Once the average aft velocity is known, the aft flow rate in cubic feet per minute is easily computed using the formula:Q=AV where:
Q=Quantity of flow in cubic feet per minute;
A=Cross sectional area of duct in square feet; and
V=Average velocity in feet per minute.
Manufacturers of aft filters, cooling and condenser coils and similar equipment often publish data from which approximate aft flow can be determined. It is characteristic of such equipment to cause a pressure drop which varies proportionately to the square of the flow rate. Manufacturers provide curves for air flow versus resistance. For example, a curve for a clean air filter might indicate an air flow of 2,000 c.f.m. for a pressure drop of 0.50 in w.c.
Additionally or alternatively, given a manufacturer's specification for a filter, the flow can be calculated. For example, a specification may state that a given flow Q (ft3/min.) occurs at a given differential “h” (inches w.c.), flow at other differentials can be determined using the formula:
      Q    n    =      Q    ⁢                            h          n                h            where:
Q=Given flow in cubic feet per minute;
Qn=Other flow in cubic feet per minute;
h=Given differential in inches w.c. (corresponding to given flow); and
hn=Differential in inches water column for other flow conditions.
The above represents some of the current, ideal practices for taking traverse pressure readings in order to help ensure accurate duct airflow measurements. Often, even more often than not, access to the ductwork by technicians is limited, which can prevent the technician from taking traverse readings at the ideal location. Many times, these locations do not allow enough room for the air flow to become fully developed, which can prevent accurate traverse readings. For example, in some scenarios, limited access to ductwork forces the technician to take the traverse reading near a “tee” in the ductwork, where separation regions can exist. In these regions, re-circulation and reverse flow can occur.
Because of this, in many cases, traverses are not suitable for determining airflow in HVAC systems due to short runs and the characteristic recirculation and reverse flow that occurs therein. Testing has proven that accurate flow measurements taken via traverse readings do not occur until they are taken at a distance beyond 7.5 diameters, i.e., at least 8.5 diameters, downstream of the tee junction. For example, for a 12-inch diameter duct, the traverse measurements must be taken at least 8.5 diameters, or 8.5 feet, from the tee junction in order to be accurate. Many times, this is not possible, as the building architecture dictates that the measurements can only be taken in closer proximity to the junction.